phonemst_bv is the driving table and irrespective of the optimizer will be a
full table scan. However, if you have index on
submast_bv.subm_account_group, submast_bv.subm_site_id, index will be used.
Does the submast_bv table have index on these (any or both) columns ?

I could simulate the same query at my end on 8.1.6 ver of Oracle by creating
index on SUBMAST and ran explain plan. It did show Index access for the
SUBMAST Table. (Non Unique Index, ofcourse!). I had CHOOSE as the
optimizer. Else, you'll need to investigate it further...